1. Field of Invention
The present invention relates to a light-emitting device.
2. Related Art
LED (light-emitting diode) is a semiconductor component and is used as the light source of indicator and outdoor media board many years ago. Compared with the traditional light source, the LED has the advantages of good efficiency, long lifetime, and strong structure, so that it has been widely used in various kinds of electronic products.
The control method for the light-emitting apparatus that uses the LED as the light source commonly includes a constant voltage control and a constant current control. As shown in FIG. 1A, a conventional light-emitting apparatus 1A, which has the constant voltage control, includes a light-emitting module 11, a capacitor 12, a plurality of resistors 13 and a constant voltage source 14. In order to ensure the signal inputted into the LED to be a constant voltage signal, the capacitor with large capacitance or complex rectifying circuit is usually configured so as to achieve the desired constant voltage. Thus, the manufacturing cost is increased.
Although the constant voltage control has the advantage of simpler layout design, it cannot provide a stable current for the light-emitting module. Since the LED emits light due to the combination of the electrons and holes to release the excess energy, the change of the applied current can induce a sufficient influence to the lighting property of the LED. In other words, the constant voltage control cannot precisely control the lighting property of the LED.
As shown in FIG. 1B, a conventional light-emitting apparatus 1B, which has the constant current control, includes a light-emitting module 11, a capacitor 12, a plurality of resistors 13, a constant current source 15, and a detecting unit 16. In this case, the constant current control can provide a more stable current to the LED. However, in practice, the forward voltages of the LEDs may have difference from each other because of the factors of the manufacturing process and operation temperature. In order to eliminate this difference, the resistor 13 must be used as the current limiter to absorb the power difference caused by the electrical variation so as to stable the current. Thus, the additional power loss may occur.
In the light-emitting apparatus of either the constant voltage control or the constant current control, a power supply unit for providing a stable power source or an element for stabling the voltage or current is necessary. Therefore, it is an important subject of the invention to provide a light-emitting device capable of automatically adjusting the number of light-emitting units of the light-emitting device to achieve the variable voltage driving in response to the fluctuation of an external power.